This invention deals with the fabrication parameters for making bistable twisted nematic liquid crystal displays. In particular, the conditions of the twist angles and the retardation of the liquid crystal cell in order to optimize its optical properties such as the contrast ratio, the viewing brightness and the viewing angle are given.
Bistable twisted nematic (BTN) displays are displays which possess at least 2 stable twist states under a zero voltage condition. The first such device was reported by Berreman in 1981. A patent on this display was issued in 1985. This BTN display has the advantage that, unlike an ordinary LCD, no holding voltage is needed to display the images. At zero volts, the alignment of the BTN can take on either of two twist states, depending on the choice of the rubbing directions of the alignment layers and the d/p ratio, where d, p are the thickness and pitch of the LC cell respectively.
The physics of the BTN lies in the interaction between the cell boundary conditions and the natural twist of the LC material. If the rubbing of the alignment layers favors a twist angle of xcfx86, and if the natural twist of the nematic LC itself is xcfx86+xcfx80, then the twist states xcfx86 and xcfx86+2xcfx80 are equally stable (or metastable). Hence, for bistability to occur, the d/p ratio of the LC cell should be xcx9c(0.5+xcfx86/2xcfx80).
The optical properties of the BTN depends strongly on the input polarizer angle xcex1, the output polarizer angle xcex3, the twist angle xcfx86, and the thicknessxe2x80x94birefringence product dxcex94n of the LC cell. So far, only a few combinations of (xcfx86, xcfx86+2xcfx80) have been successfully made, including the (xe2x88x9290xc2x0, 270xc2x0), (0xc2x0, 360xc2x0) and (90xc2x0, 450xc2x0) BTNs. However, none of these BTN possesses all the desired qualities of high brightness, high contrast, low color dispersion, and low sensitivity to dxcex94n changes (thus ease of fabrication). It is the purpose of this invention to disclose many possible combinations of (xcex1, xcex3, xcfx86, dxcex94n) that can produce BTNs with excellent optical properties and ease of fabrication.
The present invention gives the fabrication conditions of liquid crystal cells and the input/output polarizer angles in order to achieve excellent optical quality bistable twisted nematic liquid crystal displays. The conditions to be detailed are the twist angle xcfx86, the thickness of the liquid crystal cell d, the liquid crystal birefringence xcex94n, the input polarizer angle xcex1, and the output polarizer angle xcex3.